Casting a true Patronus is not about the absence of fear of death. It's about "The will to defeat death, not just for yourself but for everyone, through your own strength".

The Mirror's desire detection is unfoolable. Which means that the Confundus made Voldemort-Dumbledore actually want to see Dumbledore's family in the afterlife. Voldemort's thought-patterns leaked through, which started unraveling things the Confundus made him believe/want, but before that he did actually believe/want them.

If the Confundus can make someone really want that, it can make them really want to defeat death not just for themself but for everyone through their own strength.